Bias-selected n-p.sup.+ -n mercury-cadmium-telluride (HgCdTe) detectors that are sensitive to infrared (IR) radiation within two spectral bands, or "colors", have been fabricated using various combinations of molecular beam epitaxy (MBE) and liquid phase epitaxy (LPE). These detectors include those fabricated solely by MBE; those fabricated by the MBE growth of a long wavelength IR (LWIR) layer on top of an LPE grown mid-wavelength IR (MWIR) p.sup.+ -n double layer; and those fabricated using only LPE.
However, a structure grown using MBE typically exhibits a high leakage current and low R.sub.O A. This is due to the immaturity of the MBE growth process relative to LPE. Also, a LPE grown structure typically exhibits a poor LWIR quantum efficiency because of an inherent compositional grading that results from LPE growth. An improvement in the LWIR quantum efficiency would require growth at higher temperatures, resulting in increased Hg-Cd interdiffusion. However, this interdiffusion is known to be undesirable because it results in unwanted heterojunction transistor bipolar gain, and the unwanted detection of MWIR radiation when operating in the LWIR radiation detection mode.
Commonly assigned U.S. Pat. No. 5,113,076, "Two Terminal Multi-band Infrared Radiation Detector" to E. F. Schulte, discloses a radiation detector having two heterojunctions that function in a manner analogous to two back-to-back photodiodes. Each of the photodiodes is responsive to radiation within a different IR spectral band, such as LWIR and MWIR. Disclosed configurations include an n-p-n configuration, a p-n-p configuration, and a p-n-p-n configuration.
Reference in this regard is also made to the n-p+-n dual-band detector described by J. M. Arias et al. in the Journal of Applied Physics, 70(8), 15 Oct. 1991, pgs. 4820-4822.
This triple-layer n-p.sup.+ -n structure assumes that MWIR absorption occurs in the bottom n-type layer, and LWIR absorption in the top n-type layer. However, if this structure were fabricated by the LPE growth of Hg.sub.1-x Cd.sub.x Te, the physics of the growth process is such that the composition parameter x would decrease as the layer is grown. The result is the generation of an intrinsic electric field that aids in the collection of minority carrier holes in the MWIR responsive layer, but may hinder the collection of holes in the LWIR responsive layer. As a result, LWIR quantum efficiency is reduced.